The present invention relates to frequency multipliers. More particularly, the present invention relates to frequency multipliers for use in microwave or millimeter wavelength circuits, especially in high power transmitter systems.
There are a number of microwave systems having transmitters operating at millimeter wavelengths, e.g., 94 GHz, that require peak power outputs up to tens of watts. Cost, size, and power constraints dictate that the transmitters used in these and other applications be inexpensive, small, and energy efficient.
Coherence requirements frequently preclude the use of diodes such as Gunn or IMPATT (Impact Ionization Avalanche Transit Time) diodes to generate power directly at frequencies such as 94 GHz. In addition, if these two-terminal devices are used as amplifiers rather than oscillators in a coherent system, they require additional costly, bulky devices, for example, a ferrite circulator and its associated magnet. Further, three terminal devices, such as HEMTs (High Electron Mobility Transistors) or pseudomorphic HEMTs (PHEMTs) with gate lengths down to 0.1 .mu.m, have demonstrated useful gain at modest power levels at 94 GHz. There are, however, reliability concerns for such devices having short gate lengths and the typical low manufacturing yields increase their cost. Probably their most serious deficiency, however, is their low power output which in turn requires combining a large number of these devices to achieve the required power level. Unfortunately, power combining has generated substantial power loss and low efficiency.
Alternatively, high power millimeter wave transmitters may use a first stage having high power and efficiency by employing MESFET (Metal Semiconductor Field Effect Transistor) devices to generate the required power level at lower frequencies and a second stage having a frequency multiplier relying upon nonlinear elements such as varactors (voltage dependent capacitors) to generate the required millimeter wave frequency. In such a scheme, the frequency multiplier must have (1) high efficiency, (2) high power handling capability, and (3) good heat dissipation properties, and must be compact and manufacturable at low cost.
In addition to the aforementioned requirements, the characteristics of the frequency multiplier should facilitate design to yield input and output impedances appropriate to the circuit incorporating the frequency multiplier.
A single varactor capable of handling high power levels would require a large junction area. But, a large junction area corresponds to an extremely low impedance at millimeter wavelengths. This low impedance would result in unacceptable losses due to the addition of impedance matching input and output circuits. Furthermore, the varactor would have a narrow bandwidth and tuning would be critical. Others have addressed this characteristic of varactors (see P. W. Staecker et al., "Multi-Watt Power Generation at Millimeter-Wave Frequencies Using Epitaxially-Stacked Varactor Diodes," 1987 IEEE MTT-S Digest, pp. 917-920, Jun. 1987; J. F. Cushman et al., "High Power Epitaxially-Stacked Varactor Diode Multipliers: Performance and Applications at W-Band," 1990 IEEE MTT-S Digest, pp. 923-926, May 1990). Unfortunately, this type of structure is difficult to process and has poor thermal properties because only the bottom P-N junction varactor is in good thermal contact with the heat sink. As a result, the temperature rise of the upper P-N junctions can be hundreds of degrees greater than the bottom junction, resulting in different electrical characteristics among the P-N junctions.
In light of the foregoing, there is need for a frequency multiplier that is small and inexpensive and can handle high power with high efficiencies.